ReplicatedPG: remove pushing/pulling

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG/Backend: move prep_push and friends to ReplicatedBackend

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedBackend: remove priority from Pull/PushInfo

Instead, we'll just nab the priority from the messages.

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedBackend: track object contexts in Push/PullInfo

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedBackend: handle stats properly

Signed-off-by: Samuel Just <sam.just@inktank.com>